How to do seperate render passes in blender?
Actually, your question is a little bit vague, you have everything you need to begin to understand this topic on the Blender Reference Manual

Short answer, because this is a huge topic, and your question implies that the concept itself isn't quite clear for you (the reason I advised you to first read the manual ;) ), but the concept is to :


activate the needed passes in the render layers section





use nodes to separate the different passes




This opens a lot of possibilities, because you can independently tweak every passes, export each ones to different locations, if you have just one parameter to modify, it doesn't need to re-render everything, etc...